Lionel Bringoux

Force-field adaptation without proprioception: Can vision be used to model limb dynamics?

Because our environment and our body can change from time to time, the efficiency of human motor behavior relies on the updating of the neural processes transforming intentions into actions. Adaptation to the context critically depends on sensory feedback such as vision, touch or hearing. Although proprioception is not commonly listed as one of the main senses, its role is determinant for the coordination of daily gestures like goal-directed arm movements. In particular, previous work suggests that proprioceptive information is critical to update the internal representation of limb dynamic properties. Here, we examined the motor behavior of a deafferented patient, deprived of proprioception below the nose, to assess adaptation to new dynamic conditions in the absence of limb proprioception. The patient, and age-matched control participants, reached toward visual targets in a new force field created by a rotating platform. Full vision of the limb and workspace was available throughout the experiment. Although her impairment was obvious in baseline reaching performance, the proprioceptively deafferented patient clearly adapted to the new force conditions. In fact, her time course of adaptation was similar to that observed in controls. Moreover, when tested in the normal force field after adaptation to the new force field, the patient exhibited after-effects similar to those of controls. These findings show that motor adaptation to a modified force field is possible without proprioception and that vision can compensate for the permanent loss of proprioception to update the central representation of limb dynamics.

Influence of whole-body pitch tilt and kinesthetic cues on the perceived gravity-referenced eye level

We investigated the effects of whole body tilt and lifting the arm against gravity on perceptual estimates of the Gravity-Referenced Eye Level (GREL), which corresponds to the subjective earth-referenced horizon. The results showed that the perceived GREL was influenced by body tilt, that is, lowered with forward tilt and elevated with backward tilt of the body. GREL estimates obtained by arm movements without vision were more biased by whole-body tilt than purely visual estimates. Strikingly, visual GREL estimates became more dependent on whole-body tilt when the indication of level was obtained by arm lifting. These findings indicate that active motor involvement and/or the addition of kinesthetic information increases the body tilt-induced bias when making GREL judgements. The introduction of motor/kinaesthetic cues may induce a switch from a semi-geocentric to a more egocentric frame of reference. This result challenges the assumption that combining non-conflicting multiple sensory inputs and/or using intermodal information provided during action should improve perceptual performance.

Perception of slow pitch and roll body tilts in bilateral labyrinthine-defective subjects

The aim of the present study was to examine whether the perception of slow body tilts in total darkness was affected by a complete loss of vestibular function. Four blindfolded bilateral labyrinthine-defective subjects (LDs) and 12 normal subjects (Normals) were seated and immobilized with large straps against the back of a rotating L-shaped platform, and were passively displaced from the upright at 0.05 degrees x s(-1) in the pitch and roll dimensions. Subjects were asked to detect the slow change in their body orientation, by indicating as soon as possible the direction of tilt. After a brief period of practice observed for all LDs at the beginning of the session, results showed no significant difference between LDs and Normals in the mean detection threshold recorded for each direction of tilt. The mean perceptual threshold was 4.4 versus 5.1 degrees in the roll dimension, and 6.1 versus 6.1 degrees in the pitch dimension, for the LDs and Normals, respectively. These findings indicate that the accurate perception of body orientation in quasi-static conditions is mainly allowed by somatosensory information rather than by otolithic inputs.

Contribution of somesthetic information to the perception of body orientation in the pitch dimension

This study investigated the contribution of otolithic and somesthetic inputs in the perception of body orientation when pitching at very slow velocities. In Experiment 1, the subjects’ task was to indicate their subjective postural vertical, in two different conditions of body restriction, starting from different angles of body tilt. In the “strapped” condition, subjects were attached onto a platform by means of large straps. In the “body cast” condition, subjects were completely immobilized in a depressurized system, which attenuates gravity-based somesthetic cues. Results showed that the condition of body restriction and the initial tilt largely influenced the subjective postural vertical. In Experiment 2, subjects were displaced from a vertical position and had to detect the direction of body tilts. Results showed that the threshold for the perception of body tilt was higher when subjects were immobilized in the body cast and when they were tilted backward. Experiment 3 replicated the same protocol from a supine starting position. Compared to results of Experiment 2, the threshold for the perception of body tilt decreased significantly. Overall, these data suggested that gravity-based somesthetic cues are more informative than otolithic cues for the perception of a quasi-static body orientation.

Bayesian networks and information theory for audio-visual perception modeling

Thanks to their different senses, human observers acquire multiple information coming from their environment. Complex cross-modal interactions occur during this perceptual process. This article proposes a framework to analyze and model these interactions through a rigorous and systematic data-driven process. This requires considering the general relationships between the physical events or factors involved in the process, not only in quantitative terms, but also in term of the influence of one factor on another. We use tools from information theory and probabilistic reasoning to derive relationships between the random variables of interest, where the central notion is that of conditional independence. Using mutual information analysis to guide the model elicitation process, a probabilistic causal model encoded as a Bayesian network is obtained. We exemplify the method by using data collected in an audio-visual localization task for human subjects, and we show that it yields a well-motivated model with good predictive ability. The model elicitation process offers new prospects for the investigation of the cognitive mechanisms of multisensory perception.

Afforded actions as a behavioral assessment of physical presence in virtual environments

A particular affordance was used as a potential candidate for behavioral assessment of physical presence in virtual environments. The subjects’ task was to walk through a virtual aperture of variable widths. In the case of presence, the subjects’ body orientation, while walking, was hypothesized to be adapted to the width of the aperture and to their own shoulder width. Results show that most subjects adapted their behavior to both their body architecture and the virtual width constraints. These subjects exhibited a behavioral transition from frontal walking to body rotation while walking through broad to narrow apertures. The same behavioral transition has already been documented in real environments (Warren and Whang in J Exp Psychol Human Percept Perform 13(3):371–383, 1987). This behavioral adjustment is thus assumed to be an objective indication of presence. Beyond these results, the present study suggests that every afforded action could be a potential tool for sensorimotor assessment of physical presence.

Interaction between Reference Frames during Subjective Vertical Estimates in a Tilted Immersive Virtual Environment

Numerous studies highlighted the influence of a tilted visual frame on the perception of the visual vertical (‘rod-and-frame effect’ or RFE). Here, we investigated whether this influence can be modified in a virtual immersive environment (CAVE-like) by the structure of the visual scene and by the adjustment mode allowing visual or visuo-kinaesthetic control (V and VK mode, respectively). The way this influence might dynamically evolve throughout the adjustment was also investigated in two groups of subjects with the head unrestrained or restrained upright. RFE observed in the immersive environment was qualitatively comparable to that obtained in a real display (portable rod-and-frame test; Oltman 1968, Perceptual and Motor Skills 26 503–506). Moreover, RFE in the immersive environment appeared significantly influenced by the structure of the visual scene and by the adjustment mode: the more geometrical and meaningful 3-D features the visual scene contained, the greater the RFE. The RFE was also greater when the subjective vertical was assessed under visual control only, as compared to visuo-kinaesthetic control. Furthermore, the results showed a significant RFE increase throughout the adjustment, indicating that the influence of the visual scene upon subjective vertical might dynamically evolve over time. The latter effect was more pronounced for structured visual scenes and under visuo-kinaesthetic control. On the other hand, no difference was observed between the two groups of subjects having the head restrained or unrestrained. These results are discussed in terms of dynamic combination between coexisting reference frames for spatial orientation.

Effect of gravity-like torque on goal-directed arm movements in microgravity.

Gravitational force level is well-known to influence arm motor control. Specifically, hyper- or microgravity environments drastically change pointing accuracy and kinematics, particularly during initial exposure. These modifications are thought to partly reflect impairment in arm position sense. Here we investigated whether applying normogravitational constraints at joint level during microgravity episodes of parabolic flights could restore movement accuracy equivalent to that observed on Earth. Subjects with eyes closed performed arm reaching movements toward predefined sagittal angular positions in four environment conditions: normogravity, hypergravity, microgravity, and microgravity with elastic bands attached to the arm to mimic gravity-like torque at the shoulder joint. We found that subjects overshot and undershot the target orientations in hypergravity and microgravity, respectively, relative to a normogravity baseline. Strikingly, adding gravity-like torque prior to and during movements performed in microgravity allowed subjects to be as accurate as in normogravity. In the former condition, arm movement kinematics, as notably illustrated by the relative time to peak velocity, were also unchanged relative to normogravity, whereas significant modifications were found in hyper- and microgravity. Overall, these results suggest that arm motor planning and control are tuned with respect to gravitational information issued from joint torque, which presumably enhances arm position sense and activates internal models optimally adapted to the gravitoinertial environment.

Judging beforehand the possibility of passing under obstacles without motion: the influence of egocentric and geocentric frames of reference

Previous studies have shown that the perception of the earth-based visual horizon, also named Gravity Referenced Eye Level (GREL), is modified by body tilt around a trans-ocular axis. Here, we investigated whether estimates of the elevation of a luminous horizontal line presented on a screen in otherwise darkness and estimates of the possibility of passing under are identically related to body tilt in absence of motion. Results showed that subjects overestimated the elevation of the projected line, whatever their body orientation. In the same way, subjects also overestimated their capacity of passing under the line. Both estimates appeared as a linear function of body tilt, that is, forward body tilt yielded increased overestimations, and backward body tilt yielded decreased overestimations. More strikingly, the linear effect of body tilt upon these estimates is comparable to that previously observed for direct GREL judgements. Overall, these data strongly suggest that the perception of the elevation of a visible obstacle and the perception of the ability of passing under in otherwise darkness shared common processes which are intimately linked to the GREL perception. The effect of body tilt upon these perceptions may illustrate an egocentric influence upon the semi-geocentric frame of reference required to perform the task. Possible interactions between egocentric and geocentric frames of reference are discussed.

To transfer or not to transfer? Kinematics and laterality quotient predict interlimb transfer of motor learning

Humans can remarkably adapt their motor behavior to novel environmental conditions, yet it remains unclear which factors enable us to transfer what we have learned with one limb to the other. Here we tested the hypothesis that interlimb transfer of sensorimotor adaptation is determined by environmental conditions but also by individual characteristics. We specifically examined the adaptation of unconstrained reaching movements to a novel Coriolis, velocity-dependent force field. Right-handed subjects sat at the center of a rotating platform and performed forward reaching movements with the upper limb toward flashed visual targets in prerotation, per-rotation (i.e., adaptation), and postrotation tests. Here only the dominant arm was used during adaptation and interlimb transfer was assessed by comparing performance of the nondominant arm before and after dominant-arm adaptation. Vision and no-vision conditions did not significantly influence interlimb transfer of trajectory adaptation, which on average was significant but limited. We uncovered a substantial heterogeneity of interlimb transfer across subjects and found that interlimb transfer can be qualitatively and quantitatively predicted for each healthy young individual. A classifier showed that in our study, interlimb transfer could be predicted based on the subjects task performance, most notably motor variability during learning, and his or her laterality quotient. Positive correlations suggested that variability of motor performance and lateralization of arm movement control facilitate interlimb transfer. We further show that these individual characteristics can predict the presence and the magnitude of interlimb transfer of left-handers. Overall, this study suggests that individual characteristics shape the way the nervous system can generalize motor learning.